The present invention relates to a power steering apparatus for automobiles or the like, and more particularly to an electrically-operated power steering control system having an improved steering feel.
Automobiles or the like, which are driven by a great variety of users, are often equipped with power steering regardless of the size of the automobiles, so as to keep drivers less tired and helping them to drive safely.
Most of the power steering apparatuses so far used are of a hydraulic type. In recent years, however, an electrically-operated power steering apparatus has been suggested for its versatility of control and high energy-saving effect.
In most of the electrically-operated power steering apparatuses, a motor is used as an electrically-operated actuator which requires a great operating power, and the final operating power for auxiliary or assisting steering is obtained by reducing the output of the actuator.
As a result, in conventional electrically-operated power steering apparatuses, the auxiliary steering power is supplied through a reduction mechanism by a motor driven at high speed.
On the other hand, the motor has an inertial resistance due to the mass of the armature (rotor), and the reduction mechanism, which includes a plurality of gear trains, causing a great mechanical resistance when the output shaft thereof is to be rotated. Unless the motor is driven, therefore, the auxiliary steering power fails to work, so that a great power is required to turn a steering wheel against the inertial resistance and the mechanical resistance mentioned above. The steering wheel movement thus becomes very heavy and its stability deteriorates, which combined with the great friction resistance occurring in the unloaded operation of the motor from the output side of the reduction mechanism causes a very undesirable feel to the steering when operating the steering wheel.
The undesirable feel to the steering that occurs from the inertia of the motor in operating the steering wheel may be considerably improved by giving a differential characteristic to the control apparatus of the motor as disclosed in JP-A-55-76760.
In this conventional method, however, not only is a differential circuit additionally necessary, but the addition of the newly added differential characteristic requires a new noise-proof consideration, thus often increasing the production cost. In addition, the deterioration of the steering feel caused by the friction resistance described above is not improved at all, thus making it impossible to expect a satisfactory improvement of the steering feel as a whole. The deterioration of the steering feel due to the friction resistance presents itself in the form of an extreme reduction or extinction of stability which should otherwise be maintained at steering angles other than zero (neutral positions).
When steering power is applied by the driver to the steering wheel, the motor gives an assisting steering power thereto. Since the motor has an inertia, however, the driver is required to assist the motor by applying the steering power to the steering wheel which is equal to the inertial energy of the motor proportional to the square of the motor speed in order to accelerate the motor. It is also necessary for the driver to stop the motor through the steering wheel when stopping the motor. Especially when the driven intends to turn the steering wheel quickly, the time required for the motor to reach a required speed provides a load which gives a shock to the driver's hands. The above-mentioned control circuit using the differentiation is for compensating for this load. This method, however, ignores the acceleration and deceleration energy required of the motor, and therefore has the disadvantage of the shortage of energy to be compensated by the steering speed.
In addition, the conventional control circuits, in which the steering power is controlled only from a torque sensor and the steering power fails to correspond smoothly with the assisting power, has the shortcoming of a very inferior feel to the steering.